Although the total number of traffic accidents has decreased year after year, the number of traffic accidents caused by human error has not much decreased. One factor of traffic accidents caused by human error is sleepiness while driving. Accordingly, a technique is demanded which prevents traffic accidents from occurring by issuing a warning to drivers based on their arousal levels while driving.
For example, as Related Art 1 for determining arousal levels, there is a technique using a low frequency (LF)/high frequency (HF) index. In a case where a heartbeat signal of a driver is converted into a frequency and a relationship between the frequency and a spectral density is calculated, LF is a value obtained by integrating spectral densities in a low frequency band. HF is a value obtained by integrating spectral densities in a high frequency band. In Related Art 1, sleepiness is determined based on the ratio between LF and HF. For example, when the ratio between LF and HF is 7:3, the driver is determined to be in an arousal state, and when the ratio between LF and HF is 3:7, the driver is determined to be in a sleepy state.
However, in Related Art 1, a preset criterion for determining arousal levels, which is common to drivers, is used, and therefore, it is difficult to deal with individual differences in arousal levels of respective drivers.
In contrast, in Related Art 2, which is another technique for determining arousal levels, a technique is disclosed in which a driver is determined to be in an arousal state immediately after the driver starts driving, and a peak frequency of a spectral signal obtained predetermined hours after the start of driving is used as a frequency at a peak during arousal of the driver.    Patent Document 1: Japanese Laid-open Patent Publication No. 08-153288    Patent Document 2: Japanese Laid-open Patent Publication No. 2008-126818    Patent Document 3: Japanese Laid-open Patent Publication No. 2011-248746    Patent Document 4: Japanese Laid-open Patent Publication No. 2004-350773
However, there is a problem in the related arts described above that it is difficult to set, for each person, a threshold value for determining arousal levels.
In Related Art 2, although a driver is determined to be in an arousal state immediately after the driver starts driving, in some cases, drivers are not in an arousal state even immediately after they start driving. In addition, there may be a case where a peak frequency of a spectral signal, which is obtained predetermined hours after the driver starts driving, is not a frequency at a peak during arousal of the driver.